The invention relates to a method and apparatus for transmission line monitoring and, more particularly, to a method and apparatus of monitoring polarization mode dispersion in a fiber optic cable.
A typical transmission fiber used for fiber-optic communication exhibits cylindrical asymmetry with respect to its core due to imperfections in the manufacturing process. This asymmetry results in a fiber birefringence which causes one polarization component of an optical signal to travel at a different group velocity compared to the other (orthogonal) polarization component. Thus, a signal pulse propagating through the fiber exhibits pulse broadening due to the effect of polarization mode dispersion (PMD). Typically, the PMD, which is wavelength-dependent, may also be affected by environmental factors such as vibration or temperature changes, which result in changes in the stress or torsion of the fiber.
If there is only a first order PMD in the fiber, a single pulse propagating through the fiber will appear as two separate pulses at the output of the fiber. The time delay between the two output pulses is called the differential group delay (DGD). In general, however, there are higher orders of PMD, resulting in an output comprising many pulses along both polarization axes. These overlapping pulses arising from PMD, along with other effects such as chromatic dispersion, will lead to pulse broadening. In a transmission system, chromatic dispersion and other effects are usually corrected for by well-known techniques. However, signal pulses broadened by polarization mode dispersions, if left uncorrected, may still result in severe distortion or loss of data.
Thus, a PMD compensator is typically used in a transmission line to compensate for the PMD in the fiber, to ensure that signal pulses arriving at the receiver are properly reshaped. Since the PMD of the transmission line varies with time (e.g., due to external disturbances such as vibrations or temperature variations causing changes in the stress and torsion of the fiber) and wavelength, it is also necessary to monitor PMD in real-time in order to properly control the PMD compensator.
One problem with existing PMD measurement techniques is that an interruption of data transmission is required for PMD measurements, because an external signal source is typically used for monitoring purposes.
Embodiments of the present invention provide a method and apparatus for monitoring polarization mode dispersion (PMD) in a transmission fiber concurrent with data transmission through the fiber.
The PMD of the transmission fiber is measured by using a PMD monitor comprising a polarization switch, an optical bandpass filter, and a polarimeter. The polarization switch is connected to an input of the transmission fiber, while the optical bandpass filter is connected to the output of the transmission fiber.
At the output of the transmission fiber, a fraction of the data signal is directed to the optical bandpass filter, which allows a selected frequency range of the data signal to enter the polarimeter.
Using the polarimeter, measurements are performed to obtain parameters related to polarized signal components by scanning the optical bandpass filter through the spectrum of the data signal. These measurements are performed with the data signal set at two or more different polarization states that are non-orthogonal to each other, prior to propagating the data signal through the transmission fiber. The polarization mode dispersion in the fiber can be determined from an analysis of these parameters.